1. Field of the Invention
The present invention relates to an optical attenuator of multi-channels having a plurality of Mach-Zehnder-type waveguide elements, a plane waveguide type optical circuit, and an optical attenuation system having this optical attenuator.
2. Related Background Art
An optical attenuator using Mach-Zehnder-type waveguide elements is known (see, for example, OFC/IOOC' 93 Technical Digest, TuH4 by M. Kawachi, et al. and Japanese Patent Application Laid-open No. Hei 5-173101). As shown in FIG. 1, this optical attenuator has an input optical waveguide 2, a first directional coupler 3, a first optical waveguide 4, a second optical waveguide 5, a second directional coupler 6, an output optical waveguide 7, and an monitoring optical waveguide 8 all of which are provided on a substrate 1. The optical attenuator has also a heater 9 for regulating the temperature of the first optical waveguide 4. A light signal inputted into the input optical waveguide 2 is diverged by the first directional coupler 3 to be inputted into the second directional coupler 6 through the first optical waveguide 4 and the second optical waveguide 5, respectively. The light signal is outputted from the second directional coupler 6 to the optical waveguide 7 and the monitoring optical waveguide 8 at a predetermined diverging ratio. The diverging ratio is controlled by the temperature of the first optical waveguide 4 regulated by the heater 9. By detecting the light power outputted to the monitoring optical waveguide 8 by a light receiving element and by controlling the temperature of the first optical waveguide 4, that is, the optical path length, by the heater 9, the ratio of power P-out of the light signal outputted to the output optical waveguide 7 to the power P-in of the light signal inputted into the input optical waveguide 2, that is, the light attenuation, can be controlled.
As shown in FIG. 2, a plurality of Mach-Zehnder-type waveguide elements are provided in parallel on the common substrate to realize a multi-channel optical attenuator. The multi-channel optical attenuator shown in this figure has eight Mach-Zehnder-type waveguide elements provided in parallel on the common substrate. Each of the Mach-Zehnder-type waveguide elements has an input optical waveguide 2i, a first directional coupler 3i, a first optical waveguide 4i, a second optical waveguide 5i, a second directional coupler 6i, an output optical waveguide 7i and an monitoring optical waveguide 8i provided in the same way as shown in FIG. 1, and is provided with a heater 9i for regulating the temperature of the first optical waveguide 4i (i=1 to 8).
In addition, by providing a multi-channel output circuit for outputting multi-channel light signals (for example AWG: Arrayed Waveguide Grating) and an optical attenuator of multi-channels having Mach-Zehnder-type waveguide elements according to each output channel of this multi-channel output circuit on the common substrate, a compact plane waveguide-type optical circuit with small differences between the output powers of the channels can be realized.